Predictive microbiology for cosmetics based on physicals, chemicals and concentration parameters

Area of Science:

• Cosmetic Science
• Microbiology
• Formulation Chemistry

Background:

• Preservative systems in cosmetics are crucial for product stability and safety.
• Challenge testing (CT) is a standard method to verify preservative efficacy.
• Previous research identified ethanol, pH, and water as key factors influencing cosmetic preservation.

Purpose of the Study:

• To determine the necessity of challenge testing (CT) for cosmetic formulations.
• To establish correlations between ethanol, glycerine, and pH levels and preservative efficacy.
• To identify specific ingredient concentration thresholds that exempt products from CT.

Main Methods:

• Formulation of 16 oil-in-water cosmetic creams with varying ethanol (1-19%), glycerine (3-16%), and pH (6-11) concentrations.
• Performance of challenge tests (CT) according to ISO 11930:2012 to assess preservative system efficiency.
• Application of Design of Experiments (DOE) with a D-optimal plan to analyze the influence of formulation parameters.

Main Results:

• Ten formulations met criteria A, two met criteria B, and four failed the challenge tests.
• Ethanol concentrations above 16% generally exempt products from CT.
• Specific combinations of ethanol, glycerine, and pH (e.g., 10.5-16% ethanol with >10% glycerine) can exempt products from CT.
• Higher glycerine concentrations (>16%) also exempt products from CT.

Conclusions:

• A significant correlation exists between ethanol and glycerine concentrations, impacting the microbiological protection of cosmetic products.
• Optimizing ethanol, pH, and glycerine levels can lead to exemption from challenge testing for many cosmetic formulations.
• This research provides valuable insights for developing self-preserving cosmetic products, potentially reducing testing requirements.